1. Field
This disclosure relates generally to data processing systems, and more specifically, to cache coherency in a data processing system.
2. Related Art
Some data processing systems use multiple processors with closely coupled cache memories. A cache memory stores a subset of duplicative information that is stored in the system memory. Using a cache in the system may reduce the number of occurrences that a processor must communicate with the system memory via a system interconnect. However, the presence of various caches (or other memory devices) in a system can readily permit a data operand which has the same identifier or address to be present at various locations in the system. When the data operand is modified in one part of the system, an opportunity exists that an old version of the data operand will be stored or used. Memory coherency refers to the need of each processor in a multi-master data processing system to have access to the most recently modified data corresponding to a particular address in the memory system. The presence of differing data values for a same address value in a data processing system may lead to system errors.
To maintain memory coherency, reads and writes of information to the system memory are monitored or “snooped”. When either a memory read or a memory write of data at an address is detected, this address of the transaction is used as a snoop address. A snoop request is initiated and directed to all caches in the system to search for any address in the caches that match the snoop address. A snoop hit occurs for every match, and any needed corrective action is taken to maintain coherency of the data at the address in the cache where the snoop hit occurs.
Existing coherency mechanisms require low latency for responding to snoop requests in order to avoid impacting subsequent transactions by one or more masters in a data processing system. Consequently, with these systems, subsequent transactions can not be completed until all snoop responses have been obtain from caches within the system. This can reduce the efficiency of a data processing system.